<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <div class="flex">
      <div class="left"></div>
      <div class="main"></div>
      <div class="right"></div>
    </div>
  </body>
  <style>
    .left {
      width: 200px;
      height: 100%;
      background-color: red;
    }
    .main {
      width: 100%;
      height: 100%;
      background-color: blue;
    }
    .right {
      width: 300px;
      height: 100%;
      background-color: yellow;
    }
    .flex {
      display: flex;
      height: 1000px;
    }
  </style>
</html>
